Consumers and establishment limitations contribute more than competitive interactions in sustaining dominance of the exotic herb garlic mustard in a Wisconsin, USA forest

OUP accepted manuscript

The invasive plant Alliaria petiolata (garlic mustard) has spread throughout forest understory and edge communities in much of North America, but its persistence, density, and impacts have varied across sites and time. Surveying the literature since 2008, we evaluated both previously proposed and new mechanisms for garlic mustard's invasion success and note how they interact and vary across ecological contexts. We analyzed how and where garlic mustard has been studied and found a lack of multisite and longitudinal studies, as well as regions that may be under- or overstudied, leading to poor representation for understanding and predicting future invasion dynamics. Inconsistencies in how sampling units are scaled and defined can also hamper our understanding of invasive species. We present new conceptual models for garlic mustard invasion from a macrosystems perspective, emphasizing the importance of synergies and feedbacks among mechanisms across spatial and temporal scales to produce variable ecological contexts.

Invasive Species Removal Promotes Habitat Restoration but Does Not Immediately Improve the Condition of a Threatened Plant Subspecies

Impacts of invasive species on rare species are relevant to conservation. We studied the response of Leedy's roseroot Rhodiola integrifolia subsp. leedyi, a subspecies listed as threatened under the Endangered Species Act, to removal of the invasive species Japanese knotweed Fallopia japonica. Japanese knotweed has invaded the largest known population of Leedy's roseroot, affecting about 10% of all Leedy's roseroot in the world. Japanese knotweed shaded Leedy's roseroot, but the two did not share belowground resources because of their position on cliffs. To study this interaction and, ultimately, to restore Leedy's roseroot habitat to an open, high-light condition, we removed Japanese knotweed in a three-treatment block design. We measured Leedy's roseroot abundance, growth, and reproduction in treatment blocks and in uninvaded areas before and after treatment. Compared with uninvaded areas, Japanese knotweed invasion negatively affected Leedy's roseroot abundance, growth, and reproduction. Light interception by Japanese knotweed degraded the habitat for Leedy's roseroot. Herbicide removal of Japanese knotweed resulted in increased light and temperature compared with untreated invaded plots but did not affect Leedy's roseroot abundance, growth, or reproduction over the 2 y of our study. These results show that invasive species removal is conducive to restoring Leedy's roseroot habitat, but recovery in the subspecies may lag behind restoration of the habitat, suggesting that additional action or time may be required to restore preinvasion performance of Leedy's roseroot. Results of this study may inform restoration efforts for other systems and contribute to the literature on interspecific interactions.

Invasive plant alters community and ecosystem dynamics by promoting native predators

Placing invasion in a more complete food web context expands our understanding of species invasions to reflect the inherent complexity of ecological networks. Garlic mustard (Alliaria petiolata) has traditionally been predicted to dominate native communities through mechanisms embodied in popular hypotheses such as direct plant-plant interactions (allelopathy) and plant-herbivore interactions (enemy escape). However, garlic mustard also interacts directly with native predators by providing habitat for web-building spiders, which colonize the dry fruit structures (siliques) that garlic mustard leaves behind after it senesces. This interaction may lead to altered food web structure, resulting previously unexamined invasion consequences. This idea was tested in a field experiment including three treatments in which garlic mustard siliques were left intact (S+), removed (S-), or native species dominated and garlic mustard was absent (N). When siliques were intact, estimated insect abundance was locally reduced in invaded plots compared to native plots, but this relationship disappeared when siliques were removed. Phosphorus availability and the growth of one native plant species were both elevated in invaded plots where siliques were intact compared to plots where siliques were removed. Results indicate that garlic mustard's close association with web-building spiders initiates cascading invader impacts on the native community and ecosystem properties. This work supports recent theory suggesting that taking a broader food web perspective may help predict invasion impacts in different environmental contexts.

Community-level impacts of white-tailed deer on understorey plants in North American forests: a meta-analysis

The impacts of introduced or overabundant large herbivores are a concern for the conservation of forest plant communities and the sustainability of ecosystem function. White-tailed deer (Odocoileus virginianus) are considered ecologically overabundant in much of North America. Previous work suggests that impacts of deer overabundance are broadly negative and are consequently degrading forests at multiple ecological and taxonomic levels. However, no quantitative synthesis currently exists to verify the generality or magnitude of these impacts. Here, we report the results of a meta-analysis quantifying the effects of deer exclusion on the diversity, cover and abundance of woody, herbaceous and whole community components of forest understories in North America. In addition, we explore the relationships of environmental and experimental factors on the direction and magnitude of plant community outcomes using meta-regression. Using 119 calculated effect sizes sourced from 25 peer-reviewed articles, we constructed 10 community-specific data sets and found strongly positive diversity, cover and abundance responses of the woody community to deer exclusion, but no significant effects for the herbaceous or whole community components of forest understories. Local deer density and time since exclusion were significant moderators of both whole community and woody community richness. Local deer density also moderated the effects of deer exclusion on whole community cover. Plot area, in contrast, showed no relationship to any of the community response outcomes. We suggest that the use of inadequate diversity indices, non-native species replacement or legacy effects of chronic deer overabundance might explain why the herbaceous and whole community components of forest understories showed no diversity or cover responses to deer exclusion. We also suggest some strategies to increase opportunities for future quantitative syntheses of deer impacts on forests, including providing better access to existing and future data. Ultimately, we show that white-tailed deer have strongly negative impacts on forest understorey plant communities in North America, but these impacts are not ubiquitous for all components of the plant community.